12. Lamium L. (dead nettle)
Plants annual
(perennial herbs elsewhere), with taproots. Stems erect or ascending, sometimes
from a spreading base, sharply 4-angled, unbranched or branched, glabrous or
hairy. Lowermost leaves moderately to long-petiolate, grading into the sessile
upper leaves, the petiole unwinged or narrowly winged (in short-petiolate
leaves), with a relatively weak, unpleasant fragrance when crushed. Leaf blades
ovate to nearly circular or kidney-shaped, the margins toothed or scalloped and
sometimes also lobed, the surfaces hairy, the undersurface usually also with
inconspicuous, sessile glands. Inflorescences axillary, small clusters of
mostly 4–12 flowers per node, the flowers sessile or nearly so, a pair of
bractlets absent or inconspicuous, short, and slender. Cleistogamous flowers
often produced, these with calyces similar to those of open flowers, but
sometimes slightly shorter, and corollas that are only about as long as the
calyx lobes and never fully open. Calyces actinomorphic or nearly so (the
uppermost lobe often slightly longer than the lower lobes), lacking a lateral
projection, symmetric at the base, cylindric to narrowly bell-shaped, the tube
5-nerved, glabrous or sparsely and finely hairy in the mouth, the lobes shorter
than to longer than the tube at flowering, triangular to narrowly triangular,
tapered to slender, sharply pointed tips, but remaining soft (not spinescent),
not or only slightly becoming enlarged but often becoming papery at fruiting.
Corollas zygomorphic, pale pink to lavender or pinkish purple, rarely entirely
white, the lower lip sometimes lighter or white with purple spots or mottling,
the outer surface sparsely to densely hairy, the tube funnelform, relatively
shallowly 2-lipped, the upper lip entire or shallowly 2-lobed, concave
(appearing hooded), the lower lip spreading, with a conspicuous central lobe
that is narrowed at the base and entire to notched at the tip, the lateral
lobes short and toothlike, sometimes reduced to small convexities along the lip
margin. Stamens 4, not exserted (ascending under the hooded upper corolla lip),
the upper 2 stamens with slightly shorter filaments than the lower pair, the
anthers small, the connective short, the pollen sacs 2, angled to spreading,
purple, reddish purple, or white, densely hairy. Ovary deeply lobed, the style
appearing nearly basal from a deep apical notch. Style not exserted, equally
2-branched at the tip. Fruits dry schizocarps, separating into 2–4 nutlets,
these 1.5–2.5 mm long, narrowly obovoid to obovoid, more or less truncate at
the tip, often with thickened angles, with 2 flat sides and a rounded dorsal
face, the surface light brown to grayish brown or olive brown, often with
lighter mottling, finely pebbled, glabrous. About 40 species, Europe, Asia;
introduced widely in the New World.
The genus Lamium
was monographed by Mennema (1989), based primarily on the morphological study
of herbarium specimens. Mennema accepted only 16 total species instead of the
approximately 40 species accepted my most earlier botanists, treating most of
the complexes as single species, each divided into a series of subspecies or
varieties. Because his taxonomic conclusions did not take into account the
considerable body of biosystematic data on the genus, including the meticulous
cytological analyses of artificially produced hybrids performed by Bernström
(1955), Mennema’s classification has not been fully embraced by most subsequent
botanists.
A few of the
rhizomatous perennial species are cultivated as ornamentals and groundcovers,
notably L. album L. (white dead nettle), L. galeobdolon (L.)
Crantz (Lamiastrum galeobdolon (L.) Ehrend. & Polatschek; yellow
archangel) and L. maculatum L. (spotted dead nettle). The two species
found in Missouri are disturbance-adapted annuals that often produce
conspicuous displays of color in the spring, particularly in crop fields that
have not yet been plowed.